1. Field of the Invention
The invention relates to a device and method for overlaying onto a new colored background a subject clipped from a source image, and more particularly to a device and method of processing the source image having a subject moving against a first colored background in such a way as to improve, after overlaying, the image rendition constituted by the subject moving against the new colored background. Thus, the invention relates to any electronic device or apparatus implementing such a method of processing such as, for example, video mixers or autonomous devices for clipping or overlaying video images commonly referred to as "chroma-keyers". Such electronic devices or apparatuses are typically used, for example, in television studio equipment.
2. Discussion of the Background
Conventional overlay methods include a step of calculating a subject clipping key "KD". The corresponding video image "VD" representing the subject which has been overlaid on a new background is then given, in the case of multiplicative mixing, by the following function: EQU VD=KD.times.VS+(1-KD).times.VF,
where "VS" represents the source image which includes the subject moving against a first colored background and "VF" represents the new colored background onto which the subject is to be overlaid.
Following this multiplicative mixing, shades of the color of the first background persist around the subject overlaid on the new background. According to conventional techniques, these residual color shades are eliminated by adding, around the outline of the subject, a color that is complementary to the color of the first background. For this purpose, a specific decolorization key, which is calculated as a function of the value of the clipping key KD and is predetermined with the aid of templates, allows the adding of a complementary color to the color of the first background to the outline of the subject.
The relevant art is described in more detail with reference to FIG. 1, which is a schematic diagram of a conventional algorithm that obtains an overlaid video image representing a subject against a new background. Each pixel of the source video image "VS" includes three components "cbs", "crs", and "ys" which respectively refer to a blue color difference chrominance component, a red color difference chrominance component and a luminance component.
The luminance and two chrominance data are in a known format 4:2:2, where the sampling frequency for the luminance components is 13.5 MHZ while that for the chrominance components is 6.75 MHZ. It is necessary to interpolate the train of chrominance components cbs, crs, this being in the format 2:2, into a train of chrominance components cb, cr in the format 4:4. The interpolation operator 1 provides for this interpolation, according to traditional interpolation methods. The chrominance components of the pixels of the source video image VS are suppressed during line blankings so that the interpolated components situated at the start and at the end of a line are not disturbed by the line blanking signal.
The luminance samples, ys, of the source video image are delayed by the delay R1 and output therefrom as delayed luminance samples "y" so as to compensate for the delay suffered by the chrominance samples cbs and crs in the course of the interpolation operation 1.
The samples cb, cr, and y are applied to a "calculation of the clipping key" operator 2 for calculating the clipping key KD. As is appreciated in the relevant art, the clipping key KD has the function of distinguishing between the first colored background of the source video image and the subject itself. It is then necessary to define in a color space a volume representing the first colored background in such a way as to be able to extract the subject from the first colored background so that the subject may be overlaid against a new background. The calculation of the clipping key (KD) operator 2 is followed by a masking and filtering operator 3, which making it possible to remove part of the details which belong to the subject from being likened to the colored background. The example most often encountered is the loss of colorimetry of the eyes of a person. Introducing the masking and filtering operation makes it possible to nullify, locally, a level of the clipping. The masking and filtering operator is represented with dashed lines in FIG. 1 because, although frequently used, is not an essential component.
A final clipping key KDF is output by the masking and filtering operator 3 and is applied to an invertor 6 so as to transform the cue KDF into an inverse cue 1-KDF (where "cue" refers to an indication, or "hint" as to how the pixels are to be further processed).
A mixing operation is then carried out with the aid of two multipliers 4 and 5 and an adder 7, as will now be described. The KDF and the source video image is applied to the multiplier 4, after the source video image has been delayed in delay R2 which makes it possible to compensate for the delays corresponding to the successive operations 1, 2 and 3. Similarly, the background video VF is applied with the inverse key (1-KDF) to the multiplier 5. The respective products of the multipliers 4 and 5 are added by an adder 7. The resultant sum from the adder 7 is then multiplied in a multiplier 8 with a decolorization key KDEC.
As mentioned earlier, the decolorization key KDEC, calculated as a function of the clipping key KD, makes it possible to act on the outline of the subject so as to effect the decolorization of this outline. Finally, the desired video signal VD is output by the multiplier 8.
This conventional method suffers from numerous drawbacks. Apart from the fact that the decolorization key KDEC is predetermined with the aid of templates, which induces a correction that masks the details of the subject, the intervention of the multiplier 8 causes a degradation of the signal output by the adder 7.
As identified and addressed by the present invention, the conventional techniques result in poor quality decolorization. Consequently, a heavy outline emerges that masks the details of the subject and is a by-product of the poor quality decolorization.